I'm putting glass flowers that I cut out of stained glass on a metal mailbox. Just a few flowers maybe some leaves and stems, nothing else. Will grout stay on the mailbox since it will not be "sandwhiched" in between pieces? Like on the outside edges on the glass. I've made one before but used tan colored silicone to make it look like grout. Looks great but was the biggest sticky mess and said I wouldn't do it again. Oh and I did paint the mailbox first. I'm just thinking the grout wont stay on. Any thoughts on this?